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October 5, 2010

Thick Blanket Of Debris Found On Asteroid Lutetia

by Sam Savage

A huge asteroid visited by Europe's Rosetta probe is encased in a thick, dusty layer of debris at least 2,000 ft. deep, created by eons of impacts that have left the shattered surface with a texture similar to the Earth's Moon.

The findings are among the first to emerge from the plethora of data collected by Rosetta during its close flyby of Lutetia 282 million miles from Earth, beyond the orbit of Mars.

Details of the encounter are being presented at a conference this week of the Division for Planetary Sciences of the American Astronomical Society (AAS) in Pasadena, California.

Asteroids are ancient remnants of the Solar System's creation, existing primarily in the vast region of space between Mars and Jupiter. They can range in size from small boulders to objects hundreds of miles in diameter, and typically consist of rock or metal.

The Rosetta spacecraft utilized multi-wavelength cameras and spectrometers, magnetic field and plasma experiments, dust instruments and a radio science experiment to collect as much data as possible from Lutetia.

The probe flew by the asteroid at the relative speed of 9 miles per second at a minimum distance of 1,964 miles.

According to the data Rosetta gathered, scientists estimate that Lutetia has a mass of about 1,700 trillion tons and a volume of 650 trillion cubic meters, giving the asteroid a density of roughly 3.4 grams per cubic centimeters - similar to that of the Earth.

These figures will help scientists determine the asteroid's composition, internal structure and porosity, providing clues to its type, something Earth-based telescopes have had failed to do.

Some observations had suggested Lutetia was a very primitive body, or C-type asteroid, having changed little since its formation. However, other measurements had observed what appeared to be metals in its surface, indicating the rock might have undergone a greater degree of evolution and would be classified as an M-type asteroid.

"Only when we've had a good look at the infrared data from Rosetta will we be able to say something about that," said Dr. Rita Schulz from the European Space Agency during an interview with BBC News.

"The spectral information stored in the infrared will give us the minerals. When we clearly have these results then we can say, 'this is Lutetia's type'," she said.

The stunning pictures obtained by Rosetta in July leave little doubt about the effects the asteroid has suffered from impacts from other space rocks, which have ground up the asteroid's surface to a deep regolith, or "soil".

This is immediately apparent based on the softened outline of some craters, which are almost completely obscured in some cases by overlying debris.

"People have calculated that for an object of this size most of debris ejected from an impact would fall back on to the surface," said Dr. Schulz.

"The escape velocity of a 50km-radius body - that's Lutetia - would be 60m/s. And that would mean 90% of the material trying to get off the asteroid would fall back, and that could explain this blanket."

During its closest approach, the resolution in Rosetta's pictures was about 180 ft. per pixel, more than enough to distinguish remarkable details on the asteroid's surface.

Scientists are still counting craters, scarps, pits, ridges, and places where landslides have occurred. So far, nearly 240 boulders of minimum size 300 feet have been identified.

"Because the asteroid is so big, there is so much to find and we're still doing an inventory," said Dr. Schulz.

"For instance, we've now finished putting a grid on the asteroid to give coordinates for every feature."

Rosetta is currently moving further into the solar system, where it will approach another comet in 2014.